Carbon NanotubesCarbon Nanotubes

CNTs - OUTLINE

• Formation• Synthesis• Chemically modified CNTs• Properties• Applications• Carbon arc synthesis

• Andrzej Huczko, Hubert Lange Laboratory of Plasma ChemistryDepartment of Chemistry, Warsaw University

Formation

• Multi-walled nanotubes MWCNT– Prevention of formation of

pentagon defects• Covalent connection between

adjacent walls at the growing edge

• Saturation of dangling bonds by lip-lip interactions at the growing edge reduces grow rate leaving more time for annealing off the defects

Relaxed geometries at the growing edge of achiral double-wall carbon nanotubes. (a) The (5,5)@(10,10) armchair double tube, with no lip-lip interaction (structure AA-0, in perspectivic and end-on view), and with lip-lip interaction (structures AA-1 and AA-2).

TEM micrograph of MWCNT

Formation

• Single-walled nanotube SWCNT

• Molecular Dynamics simulation– Mixture of C (2500) and Ni (25)

atoms– Control temperature 3000 K– C random cage clusters, Ni prevents

the cage from closure– Grow of tubular structure by

collisions and annealing at lower T (2500 K)

Growth process of a tubular structure by successive collisions of imperfect cage clusters.

Double-wall CNT formation

Formation

• Single-walled nanotube SWCNT

• Gas-phase catalytic growth

– Transition metal catalysts (Co, Ni)

– C, metal and metal carbide clusters (aggregates)

– Metal carbide clusters saturated with C

– Nanotube grows out of the cluster

– Computer simulation• Ni atoms block adjacent

sites of pentagon

• Ni atoms anneal existing defects

Formation

• Single-walled nanotube SWCNT

• Gas-phase catalytic growth– Laser vaporization (diagnostics: Rayleigh scattering, OES,LIF )

• Optimum T (> 1100)

• Lower T results in too rapid aggregation of C nanoparticles

Formation

• Single-walled nanotubes SWCNT– Electrode or metallic particle surface

• Small flat graphene patches

– How the graphene sheet can curl into nanotube without pentagons?

• Spontaneous opening of double-layered graphitic patches

– Bridging the opposite edges of parallel patches

– Extreme curvature forms without pentagons

Synthesis

• Carbon arc– 1991 Iijima in carbon soot

– 1988 SEM images of MWCNTs from catalytic pyrolysis of hydrocarbons

– 1889 US patent: ‘hair-like carbon filaments’ from CH4 decomposition in iron crucible

• DC arc sublimation of anode– MWCNT

• He, 500 torr• Cathode deposit

– Outer glossy gray hard-shell– Inner dark black soft-core with

nanotubes

– SWNT• Metal catalyst (Fe, Ni, Y, Co)

– Vapor phase formation of SWCNT– Anode filled with a metal powder

• Binary catalyst– Hydrogen arc with a mixture of Ni,

Fe, Co and FeS: 1g nanotubes/hour

Synthesis

• Carbon arc MWCNT• Cathode spot hypothesis

– Materials evaporated from the anode are deposited on the cathode surface after re-evaporation by the cathode spot

• During the cooling period when cathode spot moves to the next position

• Anode spot larger and jet stronger– Mass erosion much greater

• Cathode spot weaker– Back flow of materials

Synthesis

• Carbon arc SWCNT• Occurrence

– Web-like deposits on the walls near the cathode

– Collaret around the cathode’s edge

– Soot

• Temperature control of SWCNT– Variation in conductance of the gap

– Variation in composition of Ar/He mixture• T~xHe/xAr

• Thermal conductivity of Ar 8 times smaller

– Optimal regime for maximum yield• The gap distance set to obtain strong

visible vortices at the cathode edge

– dnanotube from 1.27 (Ar) to 1.37 nm (He)

Synthesis

• Laser vaporization– Nd:YAG vaporization of graphite

• Ni, Co, 500 torr, Ar

• Majority of SWNT grow inside the furnace from feedstock of mixed nanoparticles over seconds of annealing time

TEM images of the raw soot(a) Downstream of the collector (point 2):

SWNT bundles and metal nanoparticles(b) Upstream (point 1): short SWNT (100

nm) in the early stage of growth

Synthesis

• Catalytic Chemical Vapor Decomposition CCVD (pyrolysis)– Carbon bearing precursors in the presence of

catalysts (Fe, Co, Ni, Al)

– Substrate e.g. porous Al2O3

– Example• CH4, 850-1000 °C, Al – high quality SWNT

– Large scale synthesis• Seeded catalyst

– M/SWCNT

– Benzene vapors over Fe catalyst at 1100 ºC

– Nanotube diameter varies with the size of active particles

– CNT irregular shapes and amorphous coating and catalyst particles embedded

• Floating catalyst– SWCNT

– Pyrolysis of acetylene in two-stage furnace, ferrocene precursor, sulphur-containing additive

Synthesis

• CCVD

• Conversion of CO on Fe particles– Hydrocarbons: CNTs with amorphous carbon coatings

• Self-pyrolysis of reactants at high T

– CO/Fe(CO)5 (iron pentacarbonyl)

– Addition of H2: SWNT material (ropes) yield increases 4 x at 25% of H2

collector

Synthesis

• CCVD

• HiPco High-pressure conversion of CO – Thermal decomposition of Fe(CO)5

– Fe(CO)n (n=0-4) Fe clusters in gas phase

– Solid C on Fe clusters produced by CO+COC(s)+CO2

– Rapid heating of CO/Fe(CO)5 mixture enhances production of SWCNTs

• Running conditions– pCO: 30 atm

– Tshowerhead: 1050 °C

– Run time: 24-72 h– Production rate: 450 mg/h

(10.8 g/day) SWNT of 97 mol % purity

Synthesis• CCVD - HiPco • Typical SWCNT product

– Ropes of SWCNTs

– Fe particles or clusters d=2-5 nm • SWNT d~1 nm• Nanotube stop growing

– Catalyst particle evaporates or grows too small– Catalyst particle grows to large and becomes covered with carbon

• Sidewalls of SWCNTs free of amorphous carbon overcoating

TEM images

Synthesis• CCVD – Aligned and ordered CNTs• Preformed substrates• MWNTs

– Mesoporous silica• Fe oxide particles in pores of silica

• 9% of acetylene in N2, 180 torr, 600 °C

– “Forest” on glass substrate (b)• Acetylene, Ni, 660 °C

– Catalytically patterned substrates (c)• Squared iron patterns – “Towers”

• SWNTs– Lithographically patterned silicon pillars (d)

• Contact printing of catalyst on tops of pillars

d

Pillars

Square network of SWNTs

Synthesis

• Plasma-enhanced chemical vapor deposition PECVD

• Microwave PECVD of methane– Large-scale synthesis– 600 W, 15 torr

– Mixture of CH4 and H2

– Al2O3 substrate coated with ferric nitrate solution, 850÷900 ºC

– Nucleation at the surface of Fe catalyst particles– Nanotube grows from the catalyst particle staying on the substrate surface

Tangled C nanotubes of uniform diameter (10÷150 nm), 20 m length

Synthesis

• PECVD – Microwave plasma torch– SWCNTs in large quantities (currently a few g/day, $1000/g)– Ethylene and ferrocene catalyst in atm. Ar/He– Optimum furnace temperature 850 °C– Tubular torch, Torche Injection Axiale (TIA)

Synthesis

• PECVD – DC non-transferred plasma torch

– Large-scale CNT production– 30-65 kW (100 kW), He/Ar, 200-500 torr

– C2Cl4, thoriated W cathode

• In-situ control and separation of catalyst nucleation zone

– 2-step process• Metal vapor production and

condensation into nanoparticles at a position of carbon precursor injection

• CNTs nucleation

Synthesis

• Pulsed RF PECVD– Vertically aligned CNTs

– CH4 RF glow discharge

• 100 W peak power, 53 Pa

– Ni catalyst thin films on Si3N4/Si substrates (650 °C)

– Alignment mechanism turns on by switching the plasma source for 0.1 s– Sharp transition

• Pulsed plasma-grown straight NTs

• Continuous plasma-grown curly NTs

Continuous mode pulsed mode

Synthesis

• Graphite vaporization in RF generator

• MWCNTs– Without metal catalyst– Innermost diameter down to nm

(a) the chamber with an attached plasma torch in an RF plasma generator

(b) A graphite rod in a plasma flame and the resultant deposits on the graphite rod.

Synthesis

• Hollow cathode glow discharge (Lange)– Graphite hollow cathode

• CCVD deposition >600 °C

• Carbon cold cathodes for FED’s should be deposited below strain point 666 °C

– Catalyst: ferrocene, Substrate: Anodic aluminum oxide AAO– C nanostructures

• Pillar-like, cauliflower-like, shark-tooth-like and tubular

• Amorphous fibers– Heated to 1100 °C converted into well-crystallized nanotubes

Synthesis

• Carbon arc in cold liquid– Rapid quenching of the carbon vapor– 25 V, 30-80 A, C-A gap 1 mm– Anodic arc

• Only anode is consumed

Synthesis

• Solid-state formation

• Mechano-thermal process– C and BN nanotubes– 2-step process: milling and annealing

• High-energy ball milling of graphite and BN powders

– At room temperature, N2 or Ar at 300 kPa

– Catalytic metal particles from the stain-less steel milling container

– precursors

• Isothermal annealing– Under N2 flow, T1400 ºC, tube furnace

– No vapor phase during the grow process

TEM image for the graphite sampleMilled 150 hr, heated 6 hrMetal particles at tips of some nanotubes

Grow mechanism: (a) vapor phase deposition (b) solid-state diffusion

Synthesis

• Electrolysis– Electrolytic conversion of graphite

cathode in fused salts• MWCNT

– Crystalline lithium carbide catalyst• Reaction of electrodeposited lithium

with the carbon cathode

• Cost: 10 times the price of gold

Chemically modified CNTs

• Doping– Affects electrical properties of

SWNTs• Orders of magnitude decrease of

resistance

– Intercalation • e– withdrawing (Br2, I2)

• e– donating (K, Cs)

– Substitution (hetero)• B: C35B, p-type

– Pyrolysis of acetylene and diborane

• N: C35N, n-type

• B-C-N nanotubes– Arc, graphite anode with BN and C

cathode in He

TEM images of CNTs obtained by pyrolysis of pyridine (FeSiO2 substrates)a) Bamboo shapeb) Nested conec) And other morphologiesd) Coiled nanotube (Co)

Chemically modified CNTs

• Doping– Filling with metals

• Opening by boiling in HNO3

• Filling with metal salts• Drying and calcination metal oxide

• Reduction in H2 (400 °C)

– Adsorption• Interstitial sites of SWNT bundles

– Hexagonal packing

• Electrochemical storage

– Covalent attachment

Single-wall carbon nanotube “peapod” with C60 molecules encapsulated inside and the electron waves, mapped with a scanning tunneling microscope.

Carbon fibers

• Organic polymers e.g. poly(acrylonitrile)– stretching – Oxidation in air (200-300 °C)

• Nonmeltable precursor fiber

– Heating in nitrogen (1000-2500 °C)• Until 92% C

• D = 6-10 m– 5x thinner than human hair

• Adding epoxy resin

Carbon fibers

• Dispersion of SWCNTs in petroleum pitch– Tensile strength improved by 90%– Elastic modulus by 150%– Electric conductivity increased by 340%

• CNTs dispersed in surfactant solution– A soluble compound that reduces the surface tension– recondensed in stream of polymer solution

Knotted nanotube fibers, Dfiber10

Properties

• Structure• SWCNT

– Chirality (helicity)• Chiral (roll-up) vector

– (n, m) number of steps along zig-zag carbon bonds, ai unit vectors

• Chiral angle

– Limiting cases• Armchair 30º (a)• Zig-zag 0º (b)

– Strong impact on electronic properties

21 aaC mnh

Properties

• SWCNT Ropes– Tens of SWNTs packed into hexagonal

crystals (van der Waals)

TEM image of cross-section of a bundle of SWNTs

Properties• MWCNT

– Concentric SWCNT– Each tube can have different chirality– Van der Waals bonding– Easier and less expensive to produce

but more defects– Inner tubes can spin with nearly zero

friction• Nano machines

• Mechanical properties– Elastic (Young) modulus

• > 1 TPa (diamond 1.2 TPa)

– Tensile strength• 10-100 times > than steel at a

fraction of the weight

• Thermal properties– Stable up to 2800 ºC– Thermal conductivity 2x as diamond

Axial compression of SWCNT

Properties

• Electrical properties– Electric properties ~ diameter and

chirality• Metallic (armchair, zigzag)

• Semiconducting (zigzag)

– Electrical conductivity similar to Cu– Electric-current-carrying capacity

• 1000 times higher than copper wires

• Optical properties– Nonlinear– Fluorescence

• Wavelength depends on diameter– Biosensors, nanomedicine– Remotely triggered exposives– combustion

SWNTs exposed to a photographic flash- photo-acoustic effect (expansion and contraction of surrounding gas)- ignition

Properties

• Elastic properties of SWNT– BN, BC3, BC2N (C, BN) synthesized

Model of C3N4 nanotube (8,0)N violet

Applications• Bulk CNTs

– High-capacity hydrogen storage

• Aligned CNTs– Field emission based flat-panel displays– Composite materials (polymer resin,

metal, ceramic-matrix).– Electromechanical actuators

• Individual SWCNTs– Field emission sources– Tips for scanning microscopy– Nanotweezers– Chemical sensors– Central elements of miniaturized

electronic devices

• Doped SWCNTs– Chemical sensors

• Semiconducting SWCNT: conductance sensitive to doping and adsorption

– Small conc. of NO2 NH3 (200 ppm): el. conductance increases 3 orders of mag.

– SET: single electron transistor

Field-effect transistor (FET)- much faster than Si transistors (MOSFET)- much better V-I characteristics- 4 K: single-electron transistor (SET)

Batteries used in about 60% of cell phones and notebook computers contain MWCNTs.

Applications

• Batteries– Anode materials for thin-film Li-ion

batteries• Superior intercalation medium

– Instead of graphitic carbon

• Extension of the life-time

• Higher energy density– Enhanced capacity of Li+

• Li+ enters nanotube either through topological defects (n>6-sided rings) or open end

– Fuel cell for mobile terminals• 10 x higher capacity than Li battery

• Longer life-time

• Direct conversion of oxygen-hydrogen reaction energy

• Microprocessor from CNTs

Applications

• Scanning probe microscopy (SPM)

• Atomic force microscopy (AFM)– MWNTs and SWNT single or bundles

attached to the sides of Si pyramidal tips– Direct grow of SWNT on Si tip with

catalyst particles deposited (liquid)

Applications

• Hydrogen storage– Interstitial and inside– Low cost and high capacity (5.5 wt%) at room temperature– Portable devices– Transition metals and hydrogen bonding clusters doping

• Uptake and release of hydrogen

– H adsorption increases below 77 K• Quantum mechanical nature of interaction

Potential applications• “Bucky shuttle” memory device

– K@C60+@C480

• K valence e– is transferred to C shell• C60 transfers e– to capsule (low Ei) and

out of the structure

– C60@C480 • Thermal annealing of diamond powder

prepared by detonation method• Heated in graphite crucible in argon at

1800 ºC for 1 hour

(a) TEM image(b) model with K@C60

+ in bit “0”position(c) potential energy of K@C60

+, capsule

in zero field (solid line) and switching field of 0.1 V/Å (dashed lines)

(d) high-density memory board

Potential applications

• Electro-mechanical actuators – Actuator effect: the tube increases its length by charge transfer on the tube

• Expansion of C-C bond

– Artificial muscles• Sheets of SWCNTs – bucky paper

• More efficient than natural or ferroelectric muscles

The strip actuator- Strips of bucky paper on both sides of a scotch tape- One side is charged negatively and the other positively- Both sides expand but the positive side expands more than the negative

Potential applications

• Nanoscale molecular bearings, shafts and gears– Powered by laser electric field

Powered gear

Powered shaft drives gear

Benzene teeth

Potential applications

• Nanoscale molecular bearings, shafts and gears

Planetary gear

Potential applications

• Nanobots – Quantum molecular wires

• Ballistic quantum e– transport (computers)

– Heterojunctions• Connecting NTs of different diameter and

chirality

• Molecular switches

• Rectifying diode– Introducing pairs of heptagon and

pentagon Mettallic and semiconducting nanotube junction

4-level dendritic neural tree made of 14 symmetric Y-junctions

Potential applications

• Nanobots – Chemical adsorption or

mechanical deformation of NTs• Chemical reactivity and

electronic properties

Molecular actuator- CNT nested in an open CNT The Steward platform

Potential applications

• NanobotsNanobot in-body voyage: destroying cell

Potential applications

• Nanobots

Barber nanobots